The present invention relates generally to an extruder and a method for processing rubber mixtures, and in particular an extruder for producing elastomeric products including elastomeric extruded profiles and a method for processing rubber mixtures for the production of elastomeric products.
When producing elastomeric products by extrusion, it is particularly desirable to realize a substantially uniform degree of crosslinking in the rubber mixture during the vulcanizing process. The uniform degree of crosslinking serves as a means of ensuring the quality of the elastomeric product. To realize the substantially uniform degree of crosslinking, the difference in temperature between the extruded product and the temperature of the vulcanizing medium should thus be kept small. In practice, the temperature of the extrusion product when leaving the extruder should be increased from an initial starting temperature of about 70-100° C. to a level of about 130-180° C. at the start of the vulcanizing process.
In the prior art, a shear head method is used to increase the temperature of the extrudate, wherein the temperature increase is realized through the mechanical force of a rotating shear mandrill.
In another possible method of the prior art, UHF technology can be employed. However, application of the UHF technology requires a complex recipe composition for the rubber mixture, particularly for mixtures that are using nonpolar EPDM-polymers as a basis.
From EP 798 097, an extruder is known the prior art, which is provided with a throttle driven by a back coupling control that starts from a specified temperature, density and weight of the exiting extrudate. This throttle comprises single mandrills that are projecting from the outside radially into the cylinder wall of the extruder and is driven by a cam ring located exteriorly. By varying the length of projection of these mandrills into the interior of the extruder, the shearing resistance, which is impacting on the extrudate is adjustable in continuous manner. The desired extrusion temperature can thus be controlled in dependence on the operation parameters. This arrangement is a very elaborate system, which requires not only a special extrusion cylinder, through which the engagement of the mandrills is controlled from the outside, but in addition requires a specially designed screw.
While this apparatus is designed to maintain a desired optimal temperature of the extrudate, nothing is disclosed about whether the vulcanizing temperature should be reached.
Furthermore, apparatuses are known, wherein a shear head is coupled in fixed rotative engagement with the screw, while the housing, which surrounds the extrusion cylinder in a sealing manner, is rotatable in a direction opposite that of the rotational direction of the screw. By means of the continuously adjustable rotational speed of the housing, the shear forces acting upon the extrudate and thus influencing its temperature can thus be affected in a purposeful manner. In U.S. Pat. No. 4,365,946, this method is used to increase the temperature of a rubber mixture to the vulcanizing temperature.
One of the drawbacks of the afore-described apparatuses is their complex assembly and also the need that the elements which are provided to increase the temperature of the extrudate require a toothed configuration, which would therefore prevent the reconfiguration of already existing extruders.
It would therefore be desirable and advantageous to obviate these prior art shortcomings and to provide an extruder with an extruder endless screw of an improved configuration and a method designed for increasing the temperature of the rubber mixture for the elastomeric profile sections to approximate the temperature to that of the vulcanizing temperature in order to obtain a uniform crosslinking of the rubber in the elastomeric sections.